1. Field of the Invention
The present invention relates to a method for driving an organic electroluminescent display device, which uses an organic electroluminescent light emitting element (hereinbelow, referred to as organic electroluminescent element).
2. Discussion of Background
An organic electroluminescent display device has an organic electroluminescent element sandwiched between an anode and a cathode. The organic electroluminescent element, which is sandwiched between both electrodes, has unnegligible capacitance formed therein. The organic electroluminescent element has properties similar to semiconductor light emitting diodes. When the anode side of the organic electroluminescent element is provided on a higher voltage side, and when a certain voltage is applied across both electrodes to supply a current to the organic electroluminescent element, the organic electroluminescent element emits light. Conversely, when the cathode side of the organic electroluminescent element is provided on a higher voltage side, the organic electroluminescent element does not emits light since almost no current flows. For this reason, the organic electroluminescent element is also called an organic light emitting diode in some cases.
When a constant voltage is applied across an organic electroluminescent element, the luminance of the organic electroluminescent element greatly varies, depending on a change in temperature or a change with time. However, the width of variations in the luminance of an organic electroluminescent element is small with respect to the value of currents. In order to obtain required display intensity, it is common to use a constant-current driving method wherein a constant-current circuit is provided in a drive circuit to supply a constant current to respective organic electroluminescent elements.
An organic electroluminescent display device, which has an organic electroluminescent element provided in each of pixels of matrix electrodes, is available. FIG. 10(a) and FIG. 10(b) are a schematic perspective view and a schematic cross-sectional view of the organic electroluminescent display device. There are provided a set of anode strips 2 connected to an anode or forming an anode per se, and a set of cathode strips 1 connected to a cathode or forming a cathode per se, which extend in a direction perpendicular to the anode strips. When the cathode strips 1 form a cathode per se, and when the anode strips 2 form an anode per se, an intersection between a cathode strip 1 and an anode strip 2 forms a pixel, and an organic thin film (organic electroluminescent element) 3 is sandwiched between both electrodes. In this manner, pixels, which are formed by organic electroluminescent elements, are provided in a matrix fashion and in a planar fashion on a glass substrate 6.
A technique for performing display of an organic electroluminescent display device by passive matrix addressing is explained. In explanation below, one of the set of the cathode strips 1 and the set of the anode strips 2 works as scanning electrodes, and the other works as data electrodes. The respective scanning electrodes are connected to a scanning driver, which is provided with a constant-voltage circuit. By this arrangement, constant-voltage drive is performed with respect to the scanning electrodes. The scanning electrodes are sequentially scanned so that one of the scanning electrodes is in a selected state with a selection voltage applied and the remaining scanning electrodes are in a non-selected state without the selection voltage applied. In general, the scanning electrodes are sequentially scanned to have a certain drive voltage applied to pixels from the scanning electrode at one end of the set of the scanning electrodes to the scanning electrode at the other end so that one scanning electrode has the selection voltage applied thereto in every selection period and so that all scanning electrodes are scanned in a certain period.
The data electrodes are connected to a data driver, which has a constant-current circuit provided at an output stage. Display data, which correspond to the display pattern of selected scanning electrodes, are supplied to all data electrodes in synchronization with the scanning of the scanning electrodes. A current pulse, which is supplied to data electrodes from the constant-current circuit, flows in a selected scanning electrode through organic electroluminescent elements, which are located at the intersections between the selected scanning electrode and the data electrodes.
The pixel made of an organic electroluminescent element emits light only in a period wherein the scanning electrode with that pixel connected thereto is selected and there is current supply from the data electrode. When the current supply from the data electrode stop, the light emission also stops. While a current supply is being made to the organic electroluminescent elements sandwiched between the set of the data electrodes and the set of the scanning electrodes in this manner, all scanning electrodes are sequentially scanned in a repetitive fashion. In accordance with a desired display pattern, the emission and the non-emission of light is controlled with respect to the pixels of the entire display screen.
For driving an organic electroluminescent panel, the set of the anode strips 2 and the set of the cathode strips 1 of the organic electroluminescent panel may be provided so that one of the sets works as the scanning electrodes or the data electrodes. In other words, the anode strips 2 are used as the scanning electrodes while the cathode strips 1 are used as the data electrodes. Or, the anode strips 2 are used as the data electrodes while the cathode strips 1 are used as the scanning electrodes. Both sets of the electrodes have interchangeability in terms of driving the organic electroluminescent panel. The setting of the scanning electrodes and the data electrodes may be made in consideration of the polarity of organic electroluminescent elements. Generally, it is common that the data electrodes correspond to the anode strips 2 and the scanning electrodes correspond to the cathode strips 1. Hereinbelow, explanation of the driving and the display of the organic electroluminescent display device will be made about a case wherein the cathode strips 1 works as the scanning electrodes and the anode strips 2 work as the data electrodes. In explanation below, irrespective of the upper and lower directions and the right and left directions when a viewer sees a display screen, the array of pixels that extend parallel with the scanning electrodes will be also called “row”, while the array of pixels that extend parallel with the data electrodes will be also called “column”. One wherein scanning electrodes and data electrodes are provided on an organic electroluminescent element or organic electroluminescent elements will be called an organic electroluminescent panel.
First, the scanning electrodes need to satisfy the following electric potential condition. Specifically, the potential of a scanning electrode in the selected state need to be lower than the potential of a scanning electrode in the non-selected state. For the purpose, driving is performed so that the potential of a scanning electrode in the selected state is set at ground (earth) potential so as to provide a scanning electrode in the non-selected state with a higher potential than the ground potential.
When output data are turn-on data for turning on a pixel, the data electrode relevant to that pixel on the column side is supplied with a constant current, when output data are turn-off data for turning off a pixel, the data electrode relevant to that pixel on the column side are supplied with a constant voltage equal to ground potential. In other words, the data electrodes are configured so as to be switched between a constant-current output and a constant-voltage output, depending on whether a pixel is turned on or off. The reason why a relevant data electrode is supplied with the constant current output is that the luminance is controlled by the value of a current as stated earlier.
The direction of a current, which flows in an organic electroluminescent element, is set so that the current flows from the data electrode as an anode strip 2 to the scanning electrode as a cathode strip 1 through the organic thin film 3. For this reason, the potential of the data electrodes is set so as to be higher than ground potential as the potential of a scanning electrode in the selected state.
As shown in the equivalent circuit diagram of FIG. 11, organic electroluminescent elements exhibit not only an electrical property as diodes but also a capacitive characteristic. By supplying the current to a desired pixel from the data driver having the constant-current circuit, light is emitted from the pixel made of an organic electroluminescent element, which is in a row with the selection voltage applied thereto. However, the pixels that are in non-selected rows without the selection voltage applied thereto simultaneously need to be capacitively charged.
When the number of the pixels, which are connected to one data electrode, increases according to an increase in the number of rows of the matrix forming a display screen, the current required for charging the capacitance of all pixels reaches an unnegligible value. As a result, the current that flows in a selected pixel in a row with the selection voltage applied thereto decreases to providing the luminance with a lower value than the expected value.
In order to solve this problem, there has been proposed a driving method wherein all scanning electrodes are preset at an equal potential once, or the organic electroluminescent element of each of pixels is precharged so as to have a certain potential. Presetting all scanning electrodes at an equal potential or precharging the organic electroluminescent element of each of pixels to have a certain potential will be referred to “the capacitive charge”. When a pixel is energized to emit light with the maximum luminance (a luminance of 100%) after performing the capacitive charge, the data electrode relevant to that pixel is supplied with a current over substantially the full-length of the selection period. In other words, a pixel to emit light is supplied with the current over substantially the full-length of the selection period. After that, a constant voltage is applied to the data electrode relevant to the pixel to turn off the pixel. Hereinbelow, such a driving method will be referred to as the capacitive charge driving method. The capacitive charge driving method is a driving method that includes dealing with the potential of column electrodes so as to be able to flow a desired constant current through a pixel from the start of the supply of the constant current in a broad sense.
Several kinds of driving methods have been proposed as the capacitive charge driving method. A first method is a driving method wherein when driving is switched from one scanning electrode to the next one, all scanning electrodes are set at an equal potential once, and then charging is started at the equal potential for driving (see, e.g., JP-A-9-232074, paragraph 0024 to paragraph 0032 and FIG. 1 to FIG. 4). Hereinbelow, the first driving method will be referred to as the reset driving method.
A second method is a driving method wherein a charging circuit in addition to the constant current circuit is further provided on the data driver side, and the organic electroluminescent element of each of pixels is precharged only for a certain time period. The luminance is improved by increasing the driving voltage for the organic electroluminescent element (see, e.g., JP-A-11-45071, paragraph 0022 to paragraph 0029 and FIG. 2). Hereinbelow, the second driving method will be referred to as the precharge driving method.
A third method is a driving method wherein in the idle period between a scanning period and the next scanning period, a large current flows through a data electrode to be driven in the next scanning period to charge the parasitic capacitance of the respective pixels or discharge the charge having the reverse direction (see, e.g., JP-A-2001-331149, paragraph 0014). Hereinbelow, the third driving method will be referred to as the current boost driving method.
FIG. 13 shows a basic driving waveform in a case wherein the display pattern shown in FIG. 12 is displayed on a 4×4 matrix display screen having pixels positioned in columns C1, C2, C3 and C4 and in rows R1, R2, R3 and R4. Now, a driving method wherein the time width of an output current pulse from the data driver is modified will be explained.
As shown in FIG. 13, the current pulse is supplied so as to have a pulse width occupying substantially the full width of the selection period with respect to a pixel, which is required to emit light with the maximum luminance (a luminance of 100%). The current pulse is supplied so as to have a pulse width occupying a half width in comparison with the case of a luminance of 100% with respect to a pixel, which is required to emit light with a luminance of 50%. After that, the data electrode is connected to the constant-voltage source for supplying a voltage to turn off the pixel. This driving method is called pulse width modulation (hereinbelow, also referred to as PWM).
In the conventional driving methods, pixels are actually driven after capacitive charge as stated earlier. When the voltage that is applied to the pixels at the time of completion of capacitive charge (charged voltage) fails to reach the voltage that is applied to the data electrodes at the time of driving a pixel (driving voltage), the difference between the charged voltage and the driving voltage causes a decrease in luminance in some cases. FIG. 14(a) shows an example of the applied voltage, which is applied to a pixel to emit light with a luminance of 100% or a luminance of nearly 100%. In FIGS. 14(a) and 14(b), the time period for supplying a constant current is indicated in the horizontal direction, and an applied voltage is indicated in the vertical direction. The rising edge of each applied voltage is located at the time when capacitive charge has been completed.
When the charged voltage has the same value as the driving voltage as shown in FIG. 14(a), selected pixels have a desired current immediately flowing therethrough. However, when the charged voltage is lower than the driving voltage as shown in FIG. 14(b), other pixels in the same column that are not selected also have a current flowing therethrough even after completion of capacitive charge until the applied voltage has reached the value of the driving voltage. As a result, the pixels to emit light are short of electric charges, lowering the luminance. When the driving voltage is lower than the charged voltage, the other pixels in the same column that are not selected also have a current flowing out thereof into the selected pixels even after completion of capacitive charge. As a result, the selected pixels have an excessive amount of electric charges, increasing the luminance.
Since the cathode strips 1 have a certain level of resistance, the amount of the current that flows into the cathode varies depending on the number of pixels to emit light per one row. As a result, the cathode potential varies depending on the variation of a display pattern. Even when pixels emit light with a relatively high luminance, such as a luminance of 100% or a luminance of nearly 100%, chrominance non-uniformity is caused in a horizontally striped shape according to a display pattern, depending on the variation of a display pattern and the difference between the charged voltage and the driving voltage, as shown in FIG. 15(b). This type of display state is called horizontal cross-talk. FIG. 15(b) shows a case wherein although an attempt is made to turn off a portion of the display screen and emit light from the remaining portions with a luminance of 100% as shown in FIG. 15(a), the luminance becomes darker than expected since the cathode potential in a row having a large number of pixels to turn on increases to prevent a certain level of current from flowing the organic electroluminescent elements forming the pixels to turn on.
When light emission is made with a low luminance by PWM and so on, the problem of horizontal cross-talk becomes a big issue. FIGS. 16(a) and 16(b) show examples of the applied voltage for turning on a pixel by PWM. In FIGS. 16(a) and 16(b), the time period for supplying a constant current is indicated in the horizontal direction, and each applied voltage is indicated in the vertical direction.
When the charged voltage has the same value as the driving voltage as shown in FIG. 16(a), selected pixels have a desired level of current immediately flowing therethrough. However, when the charged voltage has a different value from the driving voltage as shown in FIG. 16(b), other pixels in the same column that are not selected also have a current flowing therethrough even after completion of capacitive charge until the applied voltage has reached the value of the driving voltage. When a pixel is energized to emit light with a low luminance as shown in FIG. 16(b), the time period for supplying a current to the relevant data electrode ends before the applied voltage has reached the same value as the driving voltage. In this case, the pixel emits light with a lower luminance than a desired luminance (required luminance). When all pixels have the same current-voltage characteristics in an organic electroluminescent display device, the luminance of the device uniformly lowers over the entire screen. However, in a case wherein the pixels have different current-voltage characteristics, the respective pixels have different values of currents flowing therethrough, failing to provide a uniform luminance over the entire screen even when the pixels have the same voltage applied thereacross. The current-voltage characteristics of a pixel means the relationship between a voltage applied to a pixel and a current flowing through the pixel.
In a case wherein there are variations in the current-voltage characteristics, i.e., wherein pixels have different currents flowing therethrough by application of a voltage, a pixel emits light with the required luminance and another pixel emits light with a lower luminance in spite of that all pixels to emit light are energized so as to emit light with the same luminance by constant-current drive. This creates a problem of chrominance non-uniformity wherein the luminance varies to portion from portion to such degree that can be visually recognized.
This also created a problem that the degree of the horizontal cross-talk generated becomes greater than a case wherein desired pixels are energized to emit light with a luminance of 100% or a relatively high luminance close to 100%.
When capacitive charge is performed to all pixels of an organic electroluminescent element, additional power is required for capacitive charge. This creates a problem that even when a display pattern needs a small number of pixels to emit light, the power consumption for the pixels cannot be reduced to a lower value than the power consumption required for capacitive charge.
In order to solve these problems, the inventor has proposed an electric charge control driving method wherein a data electrode in an organic electroluminescent panel is placed in a high impedance state after a constant current is supplied to the date electrode from a constant-current circuit. In the electric charge control driving method, a driving section is set in a selection period so as to have a shorter length than the selection period, and the amount of electric charges, which are supplied to pixels in the driving section, is controlled so as to correspond to required luminance. The electric charges that have been accumulated in the capacitance of the pixels in the driving section are controlled so as to be supplied to selected pixels in a non-driving section in the selection period.
When the capacitive charge is not performed, an amount of currents that flow through the pixels in a period from start of drive to a time when an anode voltage has reached a driving voltage is small, and the luminance is lower than an expected value in that period as stated earlier. In accordance with the electric charge control driving method, it is possible to uniform the luminance amount in the selection period with respect to required luminance by controlling the amount of electric charges supplied to the pixels according to the required luminance. Thus, it is possible to reduce variations in luminance, and it is therefore possible to suppress the occurrence of horizontal cross-talk.
However, in the case of using the electric charge control driving method, it is necessary to increase the driving current and the driving voltage since the energizing time is shorter than the capacitive charge driving method. For this reason, when an organic electroluminescent display device is fabricated so as to have an operable temperature range widened in the case of using the electric charge control driving method, it is necessary to provide a driving circuit having a high output voltage.